Calculating the angle to a location - ios

Question

I have an image of an arrow that behaves like a compass to a specific location. Sometimes it works, and other times it's mirrored. So if I was facing east and the location is directly east of me, it should point up, but sometimes it points down.
#define RADIANS_TO_DEGREES(radians) ((radians) * (180.0 / M_PI))
- (void)locationManager:(CLLocationManager *)manager didUpdateHeading:(CLHeading *)heading
{
// update direction of arrow
CGFloat degrees = [self p_calculateAngleBetween:_myLocation
and:_otherLocation];
CGFloat rads = (degrees - heading.trueHeading) * M_PI / 180;
CGAffineTransform tr = CGAffineTransformIdentity;
tr = CGAffineTransformConcat(tr, CGAffineTransformMakeRotation(rads) );
[_directionArrowView setTransform:tr];
}
-(CGFloat) p_calculateAngleBetween:(CLLocationCoordinate2D)coords0 and:(CLLocationCoordinate2D)coords1 {
double x = 0, y = 0 , deg = 0, deltaLon = 0;
deltaLon = coords1.longitude - coords0.longitude;
y = sin(deltaLon) * cos(coords1.latitude);
x = cos(coords0.latitude) * sin(coords1.latitude) - sin(coords0.latitude) * cos(coords1.latitude) * cos(deltaLon);
deg = RADIANS_TO_DEGREES(atan2(y, x));
if(deg < 0)
{
deg = -deg;
}
else
{
deg = 360 - deg;
}
return deg;
}
Is this the correct way to calculate my angle with another location? Or am I missing a step? Being the arrow points directly in the opposite direction sometimes, my assumption is it's an issue with my math.

Answer 1

To calculate radians from x & y:
double r = atan(y/x);
if (x<0)
r = M_PI + r;
else if (x>0 && y<0)
r = 2 * M_PI + r;
There is not issue of dividing by 0 when X is zero because the atan function handles this correctly:
If the argument is positive infinity (negative infinity), +pi/2 (-pi/2) is returned.